Process for the production of finely divided silica

ABSTRACT

A stream of hot gas from a plasma generator is contacted with a surface of a body comprising silica to form a molten zone from which evaporation of silica occurs into the gas stream, the stream is subsequently quenched by the introduction of cooling gas, and silica in finely divided form is separated from the stream.

United States Patent Chilton et al. Feb. 15, 1972 [54] PROCESS FOR THEPRODUCTION OF [56] References Cited DI I FINELY VIDED SIL CA UNITEDSTATES PATENTS [72] Inventors: Henry Thomas Joseph Chilton, Llangollen,

l, Naff x1222 Ian George Sayce, Teddmgton, Middlesex, 3533756 10/1970HouSeman'r'lm 23/294 both of England [73] Assignee: Monsanto ChemicalsLimited, London, FOREIGN PATENTS OR APPLICATIONS England 6,716,7656/1968 Netherlands ..23/182 g 6,7 l Netherlands l [2!] Appl. No.:848,659 Primary Examiner-Edward Stern Attorney-Richard W. Sternberg andRoger R. Jones [30] Foreign Application Priority Data [57] ABSTRACT Aug.13, 1968 Great Britain ..38,674/68 A Stream of hot gas from a plasmagenerator is contacted with a surface of a body comprising silica tofon'n a molten zone [52] US. Cl ..23/294, 23/182, 204/l64, from whichevaporation of silica occurs into gas Stream 23/277 108/288 B the streamis subsequently quenched by the introduction of [51 Int. Cl. ..C01b33/18 cooling gas and Silica in finely divided form is Separated f [58]Field of Search ..23/182, 293, 294; 204/192, the stream 4 Claims, 1Drawing Figure A B 6' f A i f A i r A PArEmEnrms I972 3.542453 InventorsHenry Thomas Joseph Chi/t0" David Anthony Everest Ian George sayceAtlorne y PROCESS FOR THE PRODUCTION OF F INELY DIVIDED SILICA Thisinvention relates to a process for the production of silica in finelydivided form, more particularly to a process for the production offinelydivided silica having improved surface properties.

Finely divided silica has found many industrial applications, including,for example, use as a pigment, a filler for rubbers and plastics, and asa thickening agent for liquid organic resins. The principal methods usedhitherto for the production of finely divided silicas, based either onthe hydrolysis of sodium silicate or the oxidation of silicon halideshaving various disadvantages, and the possibility of vaporizing cheapnaturally occurring forms of silica and precipitating silica in finelydivided form from the vapor has been considered as a potentially bettermethod, Hitherto, however, the silicas produced by vaporization andprecipitation have lacked the properties required for certainapplications.

We have now found that these deficiencies can be overcome by evaporatingthe silica at the high temperatures ob tainable with a plasma torch andby controlling the conditions under which precipitation occurs. Inparticular we are able to produce silica that is very effective as athickening and thixotropic agent in organic liquids such as, forinstance, polyester resins.

The process of the invention is one for the production of silica infinely divided form, in which a stream of hot gas from a plasmagenerator is contacted with a surface of a body of material of which atleast a layer at the said surface comprises silica thus forming a moltenzone of silica from which evaporation of silica occurs into the gasstream, the stream is subsequently quenched by the introductionofcooling gas, and silica in finely divided form is separated from thestream.

A form of body from which high rates of evaporation can be obtained isone having an axially extending, open-ended passage wherein at least alining to the passage comprises silica and the body is axiallysymmetrical. The usual form of such a body is an open-ended cylinder.The stream of hot gas from the plasma generator, at a sufficiently hightemperature to melt the silica, is directed into the passage therebyforming a molten zone along the walls of the passage, which zone can beheld in place by centrifugal force on rotation of the body about theaxis.

The body may have a substantially uniform composition throughout or maybe a composite structure having an inner layer (i.e., the lining of thepassage) formed of silica and an outer sheath of some more refractorymaterial such as zirconia. The body is preferably mounted for rotationin a watercooled metal housing.

Effective quenching of the hot gas usually requires that the cooling gasshould be introduced at a flow rate at least halfthe flow rate of thehot gas stream, and in fact the dilution factor can advantageously betaken to the limit of the capabilities of the condenser system to handlethe increased volume of gas. For example, the flow rate of the coolinggas can advantageously be from to times that ofthe hot gas.

A preferred aspect of the process is that the silica is condensed in thepresence of water vapor, the latter being preferably introduced into orgenerated in the gas stream immediately following the exit from thevaporization zone.

The rate of evaporation of the silica is increased by the presence of areducing agent. This follows from the fact that in the vapor phase,silica is in equilibrium with silicon monoxide and oxygen:

2Si0 2SiO+O and the removal of the oxygen by combination with thereducing agent results in the equilibrium being shifted to the right. Apreferred way of applying this principle is to use a plasma containing areducing gas, for example, hydrogen, ammonia or methane, which may bediluted with a further gas, for example, nitrogen or argon.Alternatively or additionally the solid or liquid silica from whichevaporation takes place can contain a solid or liquid reducing agent,for example, carbon. The

use of a reducing agent in this way means that the vapor contains lessthan the stoichiometric quantity of oxygen and it is necessary to makegood this deficiency by the introduction of oxygen or other gaseousoxidizing agent before the vapor is cooled to a temperature at which anysubstantial precipitation of solid occurs.

The use of hydrogen as reducing agent generates water vapor by thereduction of silica and more water vapor may be produced by oxidation ofexcess hydrogen at the quench stage.

Whether or not a reducing agent is used, it is preferred to use acooling gas containing a gaseous oxidizing agent to ensure thesubstantial absence of lower oxides of silicon in the condensate. Airis, in any event, the quench gas most usually employed for economicreasons. Other gases can be used for quenching, however, including, forexample, nitrogen, carbon dioxide, and the inert gases such as heliumand argon.

As an alternative or in addition to generating water vapor in situ,water can be supplied as an entrainment in the quench gas, for example,as a saturated vapor or as an aerosol. Moreover, water vapor itself inthe form of steam, optionally diluted, can be used as the quench gas.

Also part of the invention is an apparatus for the production of silicain finely divided form comprising a plasma generator, an axiallysymmetrical body having an axially extending openended passagetherethrough wherein at least a lining to the passage comprises silica,the body being mounted for rotation about the axis and being disposed inrelation to the plasma generator such that in operation a stream of hotgas from the plasma generator is directed into the passage, means forrotating the body, inlet means for introducing cooling gas into a hotgas stream issuing from the downstream end of the passage, and means,downstream of the inlet means, for separating finely divided silica fromthe gas stream.

Suitable bodies can be made from, for example, particulate forms ofsilica such as ground fused quartz or quartz sand by moulding a mixtureof the particulate silica with a siliceous binding agent, for example, ahydrolyzed ethyl silicate or other such binding agent of the typecommonly used in the production of refractory articles such as mouldsfor metal casting. After moulding the body, the binding agent is set orgelled and the body is fired to cement the particles and provideadequate mechanical strength.

The arrangement at the downstream end of the passage can be a tube,which may, for example be, air or water cooled, in alignment with thepassage, the tube in turn leading to an electrostatic precipitator orbag filter. The cooling gas can be fed into the cooling tube throughjets in the wall of the tube or through a gap between the tube and thedownstream end of the passage. With this arrangement, some deposition ofsilica in finely divided form occurs in the tube, but most of theproduct is collected from the precipitator or filter.

The plasma generator may be of the DC nontransferred arc type or aradiofrequency plasma torch. In another arrangement, the molten silicamay form the anode while the plasma generator is used in the transferredare mode.

Good results have been obtained using argon-hydrogen ornitrogen-hydrogen plasmas. The plasma generator is usually started witha pure argon plasma since this gas ionizes very readily. To illustratethe range of working conditions, gas can be fed to the plasma generatorat a rate of for example 50 to 150 liters (measured at NTP) per minute,and the power supplied to the generator can be from 15 to 40 kilowatts.The temperature of the plasma may be, for example, from 3,000 to l2,000C. Rates of evaporation of from 10 to 40 grams per minute of silica havebeen obtained, giving a concentration of silica vapor in the gas streamof, for example, from 0.1 to 0.5 grams per liter. On leaving thefurnace, the gas stream can be diluted with from 100 to 400 liters perminute (measured at NTP) of gas, usually air, or an equivalent volume ofoxygen or oxygen-enriched air. The temperature of the quench gas beforeinjection is usually ambient, but can be considerably below this, forexample, as low as -l C. where the gas is air.

The invention is illustrated by the following example.

EXAMPLE The apparatus used is shown in diagrammatic section in theaccompanying DRAWING, in which A is a plasma torch, B a centrifugalliquid wall furnace and C a cooling tube.

The apparatus also included an electrostatic precipitator (not shown)connected to the downstream end of the cooling tube.

The plasma torch includes a tungsten rod cathode l, a copper anodecomprising a cup 2 and a tubular stem 3, gas inlets 4 and 5 and a nozzle6 having a radial bore 7. The furnace comprises a steel tube 8surrounded by a jacket 9 through which water can be circulated. The tubeis rotatable on bearings in an annular housing (not shown) at each end.A hollow cylindrical silica core 10 is mounted coaxially within thefurnace, being held at the ends by guard rings 11 and 12. The coolingtube has a jacket 13 through which water can be The products wereevaluated as thickening and thixotropic agents in two polyester resinsamples in comparison with a commercially available silica believed tobe made by an elec tric arc process but under otherwise unknown processconditions. The viscosities of 2 percent by weight dispersions of silicain the resin were measured after blending and standing for 24 hours,using a Brookfield viscometer. Results are given in Table 2 below.

TABLE 2.Vlsc0sity in centipolsc Resin I Resin II circulated, andjuxtaposed jets 14 for the introduction of 20 quench gas into theinterior of the tube. Quench gas can also be supplied at the gap 15between the furnace and the cooling tube. A slight positive pressure ofgas (flange bleed gas) is maintained at the gap 16 between the torch andthe furnace.

As evaporation proceeds, a cavity 17 is formed in the core.

The silica can be replenished by feeding silica in the form of rod,pellets or powder into the plasma via the bore 7.

In one series of experiments, the plasma consisted largely of argon witha small amount of hydrogen as shown in the Table 1 below. The condensersystem was operated under a slight vacuum so that air was drawn inthrough the inlets in sufficient quantity to oxidize completely siliconmonoxide and hydrogen in the furnace exit gases. Some silica condensedin the watercooled tube, but most was collected in the electrostaticprecipitator. The data given in Table 1 represent operation undernonoptimum conditions.

in condenser; E, in precipitator.

NOTE. Gas flows in litres/minute at S.T.P. Product collected (g.): A,

The superiority of the silicas produced by the present process asthickening and thixotropic agents compared with the commerciallyavailable silica is clearly shown.

In a second series of experiments, the plasma torch was fed with variousmixtures of argon and hydrogen or (after starting the torch with argon)nitrogen and hydrogen. To prevent attack of the tungsten cathode bynitrogen, the nitrogen was fed tangentially into the arc chamber whilehydrogen (or argon) was fed axially along the cathode, thus shieldingthe tungsten with an inert layer. A larger condenser and precipitatorsystem than used for the first series of experiments was installed, andthe system also included a tube fitted with baffles downstream from theelectrostatic precipitator, resulting in more efficient collection ofthe product. The power supplied to the torch was higher, giving higherrates of evaporation of silica.

Air, oxygen or a mixture of the two was injected into the condenserthrough the juxtaposed jets.

The experimental data are down in Table ,3 below:

in further series of experiments, the effect on the thickening power ofthe product of varying the volume of quench gas relative to the volumeof plasma was studied. The results given in Table 5 were obtained whenthe quench gas was introduced through juxtaposed jets, and those inTable 6 were obtained by introducing the quench gas through the gap 15(see drawing). This resulted in a radial flow of quench gas into the hotefflux from the furnace.

TABLE 3 Approximatc Total Torch gases torch Flange Quench Product (g.)Initial time, power, Core diameter of Expt. No min. Ar H; N 2 kw. Ar H2Air 02 A B C Total loss (g. core (mm.)

N OTE.Gas flows in litres/minute at S3121. Product collected: A, incondenser; B, in precipitator.

The course of experiment No. 8 is shown in greater detail in Table 4Heat out to cooling Precipitator Torch water (kw.)* temperature C.)Flange Air Ar H 2 2 3 4 Jacket Exit quench This includes heat due tocombustion 01112 7.2 k

1 Heat lost to cathode. denser.

2 Heat lost to anode.

3 Heat lost to furnace. 4 Heat lost to colu- NOTE. Gas flows inlitres/minute at S.'I.I.

The results given in Table 7 were obtained using juxtaposed jets for theintroduction of the quench gas but a silica core of modified designcontaining natural silica sand such that a longer cavity was burned inthe tube. This effectively instream, said body having an axiallyextended open-ended passage and said body being one from which a highrate of evaporation is obtained until a substantial, elongated cavity isformed in said core, said molten zone of silica being formed creased thetemperature of the exit gases and a more rapid 5 along the Walls Of thep g and being l in pl ce by Genrate of fall in temperature of the gaseson quenching was lfugal force as reslm Ofrotatmg y ofslllca, quenchingtherefore achieved. The product had exceptionally good the Streamcomammg the YP 511163 y a Qoolmg gas to thickening power. The increasein thickening power with incondens? Vaponzed Slhca and P F creaseddilution and hence Cooling of the plasma is apparent densed silica fromthe stream whereby silica in finely diVldCd from a comparison ofExperiment 14 with Experiment l5, form's obtamed' although the oxygenpresent was in each case sufficient to ox- 2. A process according toclaim 1 in which the cooling gas idize all the silicon monoxide andhydrogen in the plasma. contains water vapor.

TABLE Torch gas, Quench Thickening Duralitres/min. (L/min.) S101 power,ups.

tion, Power, vapour- Expt. No mins. H N kwatt Air 02 ised, g. Speed 2Speed 3 TABLE 6 TABLE 7 Norms ON TABLEs.(a) Thickening power: this is ascreening test based on the viscosity of a 1% w./w. dispersion of theSiO in tri-tolyl phosphate. The viscosity was measured on aFerrantiViscometer, VMA spindle, speeds 2 or 3. Tri-tolyl phosphateitself had a viscosity of 78 cps. at speed 2 and 75 cps. at speed 3, anda 1% dispersion of a commercially available thickening silica had aviscosity of 87 cps. at speed 2 and a viscosity of 88 cps. at speed 3.

3. A process according to claim 2 in which the cooling gas is introducedinto the hot gas at a flow rate of at least half that of the hot gas.

4. A process according to claim 3 in which the flow rate of the coolinggas is from 10 to 20 times that of the hot gas.

2. A process according to claim 1 in which the cooling gas containswater vapor.
 3. A process according to claim 2 in which the cooling gasis introduced into the hot gas at a flow rate of at least half that ofthe hot gas.
 4. A process according to claim 3 in which the flow rate ofthe cooling gas is from 10 to 20 times that of the hot gas.